Cotton leads Pryor by 44 percent to 42 percent in the poll. Partisan polls should always be viewed with some skepticism, and while these numbers match results from other GOP polls released in the last few weeks, Democratic polling shows Pryor in slightly better shape.

Still, Pryor hasn't been able to crack 43 percent support, and Cotton is leading or close in every poll, a sign that the two-term senator faces a tough fight for reelection.

Pryor's seat is a must-win for the GOP if they hope to recapture Senate control.

The poll of 600 likely voters was conducted by GOP firm On Message, Inc. from July 29-30 and has a margin of error of plus or minus 4 percentage points.